Analysis of Charge/Discharge Behavior of Lithium-Ion Cells Using Simple and High-Precision Capacity Measurements: Relation between Capacity Degradation Rate and Coulombic Efficiency

Abstract

A simple and high-precision charge/discharge capacity measurement system was constructed by utilizing ordinary apparatuses in order to measure large-capacity (20 Ah class) and long life cells. By using this system, the performance of cells with a practicable charge/discharge rate (C/2) can be measured on several measurement days. This system can obtain the charge/discharge capacity with 5-digit precision (i.e., an error of 10 ppm or less) by averaging the charge/discharge capacity for over 10 cycles, and this precision is required for a useful estimation of battery life during stationary use. A downward cell capacity trend every cycle was quantitatively estimated. Self-discharge, named “reversible loss”, which does not affect capacity degradation, was observed. The reversible loss likely results from electrochemical self-discharge process, such as shuttle-like mechanism. A fraction of the reversible loss can be explained by the self-discharge current according to the float charge test.